Re: [PATCH] mm/slab: avoid allocating slabobj_ext array from its own slab

[Vlastimil Babka <vbabka@suse.cz>]

On 1/26/26 09:30, Harry Yoo wrote:
> On Mon, Jan 26, 2026 at 08:36:16AM +0100, Vlastimil Babka wrote:
> /*
>  * Calculate the allocation size for slabobj_ext array.
>  *
>  * When memory allocation profiling is enabled, the obj_exts array
>  * could be allocated from the same slab it's being allocated for.
>  * This would prevent the slab from ever being freed because it would
>  * always contain at least one allocated object (its own obj_exts array).
>  *
>  * To avoid this, increase the allocation size when we detect the array
>  * would come from the same cache, forcing it to use a different cache.
>  */
> static inline size_t obj_exts_alloc_size(struct kmem_cache *s,
>                                          struct slab *slab, gfp_t gfp)
> {
>         size_t sz = sizeof(struct slabobj_ext) * slab->objects;
>         struct kmem_cache *obj_exts_cache;
> 
>         /*
>          * slabobj_ext array for KMALLOC_CGROUP allocations
>          * are served from KMALLOC_NORMAL caches.
>          */
>         if (!mem_alloc_profiling_enabled())
>                 return sz;
> 
>         if (sz > KMALLOC_MAX_CACHE_SIZE)
>                 return sz;
> 
>         if (!is_kmalloc_normal(s))
>                 return sz;
> 
>         obj_exts_cache = kmalloc_slab(sz, NULL, gfp, 0);
>         /*
>          * Random kmalloc caches have multiple caches per size, and the cache

Maybe start with something like "We can't simply compare s with
obj_exts_cache, because..."

>          * is selected by the caller address. Since caller address may differ
>          * between kmalloc_slab() and actual allocation, bump size when both
>          * are normal kmalloc caches of same size.

As we don't test the other for normal kmalloc(), anymore this now reads as
if we forgot to.

>          */
>         if (s->size == obj_exts_cache->size)
>                 return s->object_size + 1;

Why switch to size from object_size for the checks? I'd be worried that due
to debugging etc this can yield wrong results?

> 
>         return sz;
> }
>